As his name suggests, his face looks exactly like a butt. He is a classic detective who finds clues in complex situations to solve mysteries and catch criminals. He loves sweet potato pie and black tea. A polite character, he is a gentleman to women and extremely courteous to men. His weapon is a powerful fart attack released from his mouth; those hit by this toxic gas display comical facial expressions and faint in agony.

Though he is young and clumsy, his small stature allows him to be quite active at times. He is a growing character who learns to find clues on his own over time. Despite being a coward, he often steps up bravely. He is frequently caught in Butt Detective's fart attacks and suffers unintentionally. Once, while trying to stake out like a TV detective, he ate so many red bean buns and milk that he gained significant weight, becoming unrecognizable to Ms. Koala. However, as it is an omnibus series, he returned to normal in the next episode. In the Village Festival arc, he has a romantic subplot with a poodle beastman named Lily. In Season 5 Episode 7, he appears bringing a lost umbrella to the Doggy Police Station, where he meets a retired detective. In a flashback to Detective Curly's rookie days about 5 years prior, he appears as a student at the police academy, showing he could speak to some extent even then.

The male police chief of the town where Butt Detective lives. He is a Maltese. He trusts Butt Detective deeply and frequently asks for his help. He loves round objects so much that he loses his mind and chases after them instinctively whenever he sees one, which sometimes complicates matters. He is determined to catch Master Thief U.

He tends to get frustrated when Butt Detective solves a case before he does. He has strong pride as a detective, but when Butt Detective shows his genius, he joins the other detectives in clapping and praising him. He is a Doberman.

A poodle beastman. His specialty is corny jokes, though they are rarely funny.

A detective with thick eyebrows who occasionally says random, out-of-place things.

A detective who appears to be overflowing with confidence.

The owner of the Lucky Cat Cafe, Butt Detective's favorite tea house. A Maneki-neko beastman.

He primarily steals beautiful treasures from all over the world. He loves roses, and because his eyes are blue, he failed to change his eye color during disguises and was caught by Butt Detective; since then, he covers his eyes when disguising. However, he still leaves flaws and keeps getting caught. He occasionally speaks French, using words like 'Très bien' or 'Adieu'.

Master Thief B, the partner and colleague of Master Thief U.